“As an honest first step to the American public, we urge you to immediately and voluntarily commit to transparency and disclose the price of your prescription drugs in direct-to-consumer advertisements,” the letter sent Friday states.

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The letter was sent to major pharmaceutical companies such as Eli Lilly, Novartis and Pfizer.

Trump released a plan last week directing the Food and Drug Administration to study the possibility of including the list prices of drugs in direct-to-consumer advertising. The measure was part of Trump's blueprint aimed at lowering prescription drug costs.

Durbin introduced a bill last year that would require a similar disclosure of drug prices.

While pharmaceutical companies have to include warnings and side effects for drugs in television advertisements, they currently do not have to include prices.

Some have voiced concerns that including the list price of drugs could confuse consumers or deter them from purchasing the product, since the price at a pharmacy is often much lower if the individual has health insurance.

At an event with reporters Tuesday, Lori Reilly, an executive vice president of the Pharmaceutical Research and Manufacturers of America (PhRMA), discussed the issue of forcing companies to include the price of drugs in ads, arguing "it’s an open question about whether or not it’s constitutional.”